It has long been known in the field of optical detectors that it is possible to simulate the effect of a single large telescope by properly combining the outputs from a number of small telescopes. FIG. 1 illustrates schematically a large reflecting telescope in which a beam of incident radiation labeled 80 strikes a primary mirror 110 and is focused down and deflected from a turning mirror 120 onto a final lens 130. The image is formed on detector 140 which is illustratively an array of photodiode detectors.
If, for example, mirror 110 were replaced by two smaller mirrors denoted 112 and 114 having the same curvature as the corresponding portions of mirror 110, these two small mirrors would reflect radiation along the dotted lines through lens 130 and down to detector plane 140. The radiation traveling along paths 112' and 114' will, of course, not be along a single line but will occupy a finite extent in space. In particular, the radiation from the two mirrors 112 and 114 will overlap when it strikes detector 140. It is therefore essential, as is known in the art, that the combination of two smaller mirrors be done in such a manner that the image is not blurred or otherwise distorted. It has been known, for example, that the "piston" effect, in which the optical path difference between mirrors 112 and 114 is not properly compensated for, may spoil the resolution. It is also known that the individual optical systems of the subapertures must transform the incoming radiation in the same manner, so that rays coming into the total aperture having the same slope and height with respect to the overall optic axis must exit the optical system, i.e., exit lens 130, also having the same slope and height.